1. Field of the Invention
The present invention relates to a circularly polarized wave antenna used for performing communication between a geostationary satellite and a mobile station.
2. Description of the Related Art
In mobile units, such as automobiles, in a system for communicating with a geostationary satellite or receiving broadcasts, circularly polarized waves are generally utilized. Accordingly, there is a demand for a small circularly polarized wave antenna for obtaining stable circularly polarized waves over a wide range of wave angles.
FIGS. 18A and 18B illustrate a typical example of a conventional circularly polarized wave antenna 101. More specifically, FIG. 18A is a perspective view of this antenna, and FIG. 18B is a side view of this antenna. The circularly polarized wave antenna 101 is formed of a ground plate 102 and four conductors 103. Each conductor 103 is formed by extending the central conductor of a coaxial cable 104. The external conductor of the coaxial cable 104 is soldered to the ground plate 102, as indicated by a soldering portion 105. Accordingly, each conductor 103 is fixed on the ground plate 102 in a cantilever form. The conductors 103 are disposed on the ground plate 102 with equal distances d, and tilt in the predetermined directions at a predetermined angle xcex1.
In the circularly polarized wave antenna 101 constructed as described above, mutually in-phase power is supplied to the four conductors 103 so as to generate a spatial phase difference of 90xc2x0. Accordingly, a main beam is directed at a certain wave angle, and a circularly polarized wave is radiated in the direction of the wave angle. Also, a conical-surface pattern at the wave angle becomes non-directional. That is, the directivity of the circularly polarized wave antenna 101 becomes as shown in FIG. 19 as viewed from any azimuth angle. When a geostationary satellite 107 is positioned at a line extending from an inclined line 106, the circularly polarized wave antenna 101 can always be directed at the geostationary satellite 107 regardless of the direction in which a mobile unit on which the circularly polarized wave antenna 101 is mounted is moved. It is now assumed that the target wave angle ranges from 30xc2x0 to 60xc2x0. In this case, if the tilting angle xcex1 of the conductor 103 is set to be about 45xc2x0, the length L of the conductor 103 is set to be about 0.65xcex0, and the distance d between the two opposing conductors 103 is set to be about 0.33xcex0 (where xcex0 indicates the free space wavelength), the optimal directivity for the above-described range of the wave angles can be obtained.
In the above-described conventional circularly polarized wave antenna 101, the four conductors 103 are disposed on the ground plate 102 with the equal distances d while being tilted at about 45xc2x0, and mutually in-phase power is supplied to the conductors 103. With this configuration, a phase shifter is not required for supplying power, and thus, the configuration of the circularly polarized wave antenna 101 can be simplified. However, as discussed above, since the four conductors 103 (having a length of approximately 0.65xcex0) are disposed with the equal distances d (approximately 0.33xcex0) at about 45xc2x0, the overall dimensions of the antenna 101 result in 0.33xcex0xc3x970.33xcex0xc3x970.46xcex0. If the frequency of 2.3 GHz (xcex0=130 mm) is used, the overall dimensions of the antenna 101 increase to 43 mmxc3x9743 mmxc3x9760 mm. Thus, the antenna 101 is not small enough to be used as a vehicle-mounted antenna. Additionally, since the conductors 103 are fixed to the ground plate 102 only in a cantilever form, they are not mechanically strong. Accordingly, due to vibrations generated in an automobile, the distances d between the conductors 103 may be changed, resulting in increased variations in the characteristics of the antenna 101, or a large stress may be applied to the soldering portions 105 of the external conductors of the coaxial cables 104 so as to cause a poor connection between the coaxial cables 104 and the ground plate 102.
Accordingly, in view of the above-described background, it is an object of the present invention to provide a compact, vibration-resistant circularly polarized wave antenna.
In order to achieve the above object, according to the present invention, there is provided a circularly polarized wave antenna including a quadrilateral columnar member mounted on a printed circuit board. Four radiation conductors are provided on corresponding side surfaces of the dielectric member while tilting in predetermined directions. The bottom ends of the radiation conductors are electrically connected to the printed circuit board, and mutually in-phase power is supplied to the four radiation conductors.
With this configuration, since the four radiation conductors are provided on the corresponding side surfaces of the quadrilateral columnar dielectric member, the length of the radiation conductors can be decreased due to the wavelength reduction factor as a result of the dielectric constant of the dielectric member. Thus, the size of the circularly polarized wave antenna can be significantly reduced. Additionally, the radiation conductors are mechanically orthogonal to each other by the dielectric member, thereby reducing variations in the characteristics or a poor connection caused by external vibrations.
In the aforementioned circularly polarized wave antenna, a through-hole extending in the axial direction may preferably be provided at the center of the dielectric member. Accordingly, the dielectric member can be lighter, and the axial ratio of circularly polarized waves at a desired frequency can be reduced. In this case, the through-hole may be formed in any shape, such as in a quadrilateral shape or in a circular shape when viewed from above, as long as it is symmetrical with respect to the axial line of the dielectric member. If the through-hole is formed in a quadrilateral shape when viewed from above, dimensional variations in molding the dielectric member can be reduced since the through-hole is similar to the outer configuration of the dielectric member.
In the aforementioned configuration, an adjusting portion may be disposed in the through-hole, and a predetermined resonant frequency may be set by adjusting the size or the mounting position of the adjusting portion. With this arrangement, variations in the antenna characteristics caused by dimensional errors of the dielectric member can be easily corrected. Thus, the resonant frequency can be easily set to a desired frequency, and the manufacturing yield can be substantially improved.
For example, the adjusting portion may be a dielectric block which is inserted into the through-hole and is mounted on the printed circuit board. Then, the resonant frequency of the circularly polarized wave antenna can be increased by decreasing the thickness of the dielectric block. Thus, if the resonant frequency is set to a value slightly lower than the desired frequency in advance, the desired resonant frequency can be easily and reliably obtained simply by decreasing the thickness of the dielectric block to a suitable value. Alternatively, the through-hole may be circular when viewed from above, and a screw thread may be formed on the inner wall surface of the through-hole, and the adjusting portion may be a dielectric male screw to be screwed into the screw thread. In this case, the resonant frequency decreases as the dielectric male screw is inserted into a lower portion of the through-hole. In contrast, the resonant frequency decreases as the dielectric male screw is inserted into a higher portion of the through-hole. Thus, the resonant frequency can be easily and reliably set to the desired frequency only by adjusting the screwing position of the dielectric male screw to a suitable position.
In the aforementioned configuration, the dielectric member may include a columnar hole extending in the axial direction at the center of the bottom of the dielectric member and may also include an adjusting recessed portion at the center of the top of the dielectric member. The predetermined resonant frequency may be set by adjusting the depth of the adjusting recessed portion. In this case, with a deeper adjusting recessed portion, the resonant frequency of the circularly polarized wave antenna becomes greater. Thus, the resonant frequency is set to a value slightly lower than the desired frequency in advance. Then, the desired resonant frequency can be easily and reliably obtained simply by adjusting the depth of the adjusting recessed portion to a suitable value. As a result, the manufacturing yield can be considerably increased.
In the aforementioned configuration, a plurality of through-holes extending parallel to the axial direction of the dielectric member may be provided. With this arrangement, the dielectric member can be lighter, and the axial ratio of the circularly polarized waves at the desired frequency can be decreased. In this case, the plurality of through-holes may be formed in any shape, such as in a quadrilateral shape or in a circular shape when viewed from above, as long as they are provided so that they are positioned symmetrically with respect to the axial line of the dielectric member, and the number of the plurality of through-holes is determined so that they are positioned symmetrically with respect to the axial line of the dielectric member.